The present invention relates generally to methods for adding alloying ingredients to molten steel and more particularly to a method for adding such ingredients in the form of shot.
Examples of alloying ingredients which are added to steel as shot are lead and bismuth. In a typical operation in which lead or bismuth is added to steel as shot, a heat of molten steel is contained in a ladle, and a pouring stream of molten steel is flowed from the ladle to a casting mold, e.g., an ingot mold. Lead or bismuth shot is directed into the pouring stream between the ladle and the casting mold or at the location where the pouring stream impacts in a partially filled casting mold.
The shot may be directed into the molten steel with a shot-adding gun through which the shot is pneumatically conveyed or through which the shot passes in a free flowing fashion, i.e., by gravity. The gun propels the shot so that it can penetrate a pouring stream of molten steel.
Among the prior art which teaches adding bismuth shot to steel, Bhattacharya et al., U.S. Pat. No. 4,255,187 teaches that the bismuth shot should have a size finer than 40 mesh (0.425 mm), and Holowaty et al. U.S. Pat. No. 4,244,737 teaches that the bismuth-containing shot should have a size finer than about 10 mesh (1.98 mm), preferably in the range 20-40 mesh (0.85-0.425 mm) with no greater than 5% minus 100 mesh (0.15 mm).
A commercially available bismuth shot heretofor utilized in conventional operations for adding bismuth to steel had a size range as follows: +18 mesh (1.0 mm), 27.9 wt.%; +20 mesh (0.85 mm), 26.0 wt.%; +40 mesh (0.425 mm), 44.8 wt.%; and -40 mesh, 6.7 wt.%.
Problems arose when bismuth or lead-containing shot of the type described above was added to steel. The recovery of the alloying ingredient contained in the shot was low, and the distribution of the alloying ingredient from one part of the heat to another was relatively non-uniform. In other words, in an operation in which the heat of molten steel was teemed into a multiplicity of ingot molds, there was a substantial variation in the percent of alloying ingredient from one ingot to the next.
It is desirable that the content of the alloying ingredient, such as bismuth or lead, be uniform from one part of the heat to another. Improved recovery is also desirable because the cost of adding an alloying ingredient to the steel decreases as recovery improves.
Oftentimes, the addition of alloying ingredients to the pouring stream generates fumes in the atmosphere adjacent the casting mold, and such fumes (e.g., lead fumes) are undesirable from a health or environmental standpoint. Accordingly, it is conventional to provide apparatus, such as an exhaust hood and associated equipment, for exhausting the atmosphere in the space adjacent the casting mold during the time the pouring stream flows from the ladle into the casting mold. When the alloying ingredient is in the form of shot of the conventional type described above, there is a significant loss of alloying ingredient to the exhaust.
Shot of the conventional type described above has a tendency to agglomerate or cake, particularly in moist or cold weather. This causes malfunctions in the shot-adding gun and non-uniform flow through the gun in turn resulting in non-uniform distribution of alloying ingredient from one part of the heat to another.